Choose the sentence that is correctly punctuated.
A. The USDA is studying weeds that are resistant to common herbicides, or "'superweeds"'.
B. The USDA is studying weeds that are resistant to common herbicides, or 'superweeds'.
C. The USDA is studying weeds that are resistant to common herbicides, or "superweeds."
D. The USDA is studying weeds that are resistant to common herbicides, or "superweeds".